vactrols and optoisolators

[GhostofJohnToad]

Got a couple MBs ready to build (double flush, Nautilus, afterlife) that require vactrols and it got me to thinking what qualities does one look for in selecting a vactrol/optoisolator for a given project?  I see mouser carries optek varieties for a lot cheaper than those that smallbear sells.  Will any of these work?  i.e.:

http://www.mouser.com/ProductDetail/Optek/OPI110/?qs=sGAEpiMZZMteimceiIVCB%252bICDjkdLUgeHSQTv5yQ18w%3d

[stecykmi]

the particular part you posted will not because it's a different type of optoisolator, having an output that's a transistor rather than a resistor like the one's we use. this means the output is basically a current source (or sink) rather than a (dynamic) resistance.

projects could probably be reworked to use this type, but they are not part-for-part compatible. optoisolators like this are most commonly available in DIP or SMT packages as well. The H11F1 that the 1776 Effects Multiplex uses is one example. Josh very cleverly uses one like a vactrol in that circuit.

[midwayfair]

One huge problem with those is that in the audio path they will usually create distortion. I've gotten them to function in many projects, but the don't sound good.

Now, there might be a clever way to use them such that they aren't in the audio path but still work right. But you can't just drop them into a design that requires an LED/LDR combo.

There are a few things to consider when deciding what optocoupler to use in any given project:

1) What's it supposed to be doing?
2) What's the maximum and minimum resistances required to make it do that?
3) What's the current draw in the circuit, so that you can tell what the minimum light resistance will be? (More current = brighter LED = less resistance.)
4) Does the circuit require matching? Optocouplers tend to be much more consistent than most LDRs.
5) Are there light history/switch on time considerations? You typically want a slower turn on time for a compressor, though some interesting things can happen when you don't.

The Double Flush is really not picky. The Neutron and my Cardinal tremolo are. The Afterlife is just "different" but definitely works better with the high dark resistance you can get out of the VTL5C3 and VTL5C1. The Magnavibe and similar circuits aren't picky but will sound much better. And so forth.

Helpful recent topic on DIYSB:
http://www.diystompboxes.com/smfforum/index.php?topic=101917.0

[GhostofJohnToad]

Great insight, I appreciate the education.  There's always some weird component pop up that I'm not familiar with that I want to know more about.